1. Field of the Invention
The present invention relates to a circularly polarized wave antenna device used for e.g., communication equipment of a mobile unit.
2. Description of the Related Art
Satellite communication using an artificial satellite is being utilized in aircraft, cars, etc. and employs circularly polarized radio waves in order to eliminate regional difference. Particularly, small-sized circularly polarized wave antenna devices are required as antennas for radio equipment using circularly polarized waves, such as GPS (Global Positioning System), DAB (Digital Audio Broadcast) using S-band, ETC (Electrical Toll Collection) or the like. To meet this requirement, the present applicant has proposed a surface-mount circularly polarized wave antenna device and radio equipment using the same, in Japanese Patent Application Publication No. 2000-183637. FIG. 11 shows the circularly polarized antenna proposed in the above-mentioned patent application.
In FIG. 11, the circularly polarized antenna has a flat-plate shaped substrate formed of a dielectric body. On one main surface of this substrate 1, radiation conductor 2 which has a substantially rectangular shape in a plan view, and of which two diagonally opposite comer portions are cut off, is formed, while, on the other main surface thereof, a ground conductor 3 is formed substantially over the entire surface, except for a wraparound portion of a feeding conductor as described later. On one side surface of the substrate 1, there is provided a strip-shaped feeding conductor 4 which extends from the main surface on which the ground conductor 3 is formed, to the main surface on which the radiation conductor 2 is formed, and each of the ends of the feeding conductor 4 are formed so as to wrap around one of the main surfaces. On opposite sides of the feeding conductor 4, capacitive loading conductors 5 and 6 are formed substantially over the entire remaining surfaces while securing electrical isolation from the feeding conductor 4, and these capacitive loading conductors 5 and 6 are connected to the ground conductor 3.
In the circularly polarized wave antenna device with these features, a stray capacitance is formed between the feeding conductor 4 and the radiation conductor 2, while a load capacitance or an electrostatic capacitance is formed between each of the capacitance loading conductors 5 and 6 and the radiation conductor 2. In this case, since the corner portion of the radiation conductor 2 on the capacitive loading conductor 6 side is cut off, a load capacitance or an electrostatic capacitance between the capacitive loading conductors 6 and the radiation conductor 2 is smaller than that between the capacitive loading conductors 5 and the radiation conductor 2 .
When the power of a transmitting signal is supplied to the feeding conductor 4, a resonant current in a linear polarization mode does not flow through the radiation conductor 2, but resonant currents separated into two resonant circuits, that is, a high-frequency resonant circuit formed by the radiation conductor 2 and the capacitive loading conductor 5, and that formed by the radiation conductor 2 and the capacitive loading conductor 6, in other words, resonant currents in a degeneration-separated mode flow through the radiation conductor 2. These two resonant currents in the degeneration-separated mode have a predetermined phase difference xcex81, generate two radiation electric fields having mutually different frequencies (f1 and f2), and radiate circularly polarized electromagnetic waves from the radiation conductor 2 in the normal direction thereto.
However, in the circularly polarized wave antenna with the above-described features, the width L of the capacitive loading conductors 5 and 6 with respect to the length of the edge 2a of the radiation conductor 2 is large, so that the paths through which two resonant currents in the degeneration-separated mode flow, depends on the width L of the capacitive loading conductors 5 and 6, the width L determining the load capacitance or electrostatic capacitance between the radiation conductor 2 and each of the capacitive loading conductors 5 and 6. As a consequence, two radiation electric fields in the degeneration-separated mode do not have a phase difference of 90xc2x0 therebetween, and the two radiation electric fields do not spatially intersect each other orthogonally. This results in elliptically polarized waves, and causes deterioration of the antenna characteristic.
Also, since the feeding conductor 4 and each of the capacitive loading conductors 5 and 6 are close to each other, the electromagnetic coupling between the feeding conductor 4 and each of the capacitive loading conductors 5 and 6 becomes large, and thereby the power of transmitting/receiving signals using the radiation conductor 2 becomes small, so that it is necessary accordingly to increase the power of transmitting/receiving signals to be supplied to the feeding conductor 4.
Furthermore, under the condition that the dielectric constant of the substrate 1 is constant, when the area of the capacitive loading conductors 5 and 6 is increased, the load capacitance or electrostatic capacitance between the radiation conductor 2 and each of the capacitive loading conductors 5 and 6 becomes large, so that the resonant frequency in the degeneration-separated mode decreases. This causes a problem in that a desired frequency cannot be obtained.
The present invention has been achieved to solve the above problems, and an object of the present invention to provide a circularly polarized wave antenna device which has improved the orthogonality of two radiation electric fields in the degeneration-separated mode.
In order to achieve the above-described object, the present invention uses the following configurations to solve the above-described problems. The circularly polarized wave antenna device in accordance with a first aspect of the invention comprises a substrate formed of dielectric material; a radiation conductor having a quadrangular shape in plan view, the radiation conductor being formed on one main surface of the substrate; a ground conductor formed on the other main surface of the substrate, the other main surface being opposed to the radiation conductor; and a feeding conductor formed on the substrate so as to extend from the other main surface toward the one main surface. In this circularly polarized wave antenna device, the radiation conductor is formed into a shape wherein the electric lengths in two orthogonal directions on the radiation conductor are equal to each other. On the substrate, capacitive loading conductors which generate load capacitance between the radiation conductor and the capacitive loading conductors, are provided at positions in the diagonal directions on the radiation conductor, the load capacitance determining the frequency difference between two resonant currents flowing through the radiation conductor.
The circularly polarized wave antenna device in accordance with a second aspect of the present invention comprises a substrate formed of dielectric material; a radiation conductor formed on one main surface of the substrate; a ground conductor formed on the other main surface of the substrate, the other main surface being opposed to the radiation conductor; and a feeding conductor formed on a side surface of the substrate so as to extend from the other main surface toward the one main surface. In this circularly polarized wave antenna device, the radiation conductor is formed into a square shape in a plan view, or an electrical square shape in a plan view. On the substrate, capacitive loading conductors which are formed between the ground conductor and the radiation conductor, and which have mutually different shapes between one of the diagonal direction and the other thereof, are provided at the extended positions of the two diagonal lines on the radiation conductor or in the vicinity thereof.
In the circularly polarized wave antenna device in accordance with the present invention, the configuration may be such that the substrate is formed into a hexahedron having two main surfaces and four side surfaces; that each of the capacitive loading conductors is disposed on the side surface on which the feeding conductor is provided, along the edge line between the above-mentioned side surface and adjacent side surface adjacent thereto, and that the length of one of the capacitive loading conductors of which one end is connected to the ground conductor, is made shorter than that of the other of the capacitive loading conductors; and that, on the side surface opposite the side surface on which the feeding conductor is provided, capacitive loading conductors which have the same length as that of the capacitive loading conductors in the diagonal directions on the main surface, are each disposed along the edge line between the side surface and adjacent side surfaces.
Also, in the circularly polarized wave antenna device in accordance with the present invention, preferably, each of the capacitive loading conductors is formed by dividing it into a plurality of capacitive loading conductor pieces with gaps interposed therebetween.
Further, in the circularly polarized wave antenna device in accordance with the present invention, it is preferable that the radiation conductor have radiation conductor extension pieces each extending downward from a comer portion of the radiation conductor along the edge line between adjacent side surfaces; and that the radiation conductor extension pieces be formed so as to have different gaps between the radiation conductor extension pieces and the capacitive loading conductors, between the two different diagonal directions.
Moreover, in the circularly polarized wave antenna device in accordance with the present invention, preferably, at least one of the capacitive loading conductors is formed so as to extend to the main surface on which the radiation conductor is formed.
Furthermore, in the circularly polarized wave antenna device in accordance with the present invention, preferably, each of the capacitive loading conductors is formed into a meander shape.
Also, in the circularly polarized wave antenna device in accordance with the present invention, preferably, the substrate is formed into a rectangular parallelepiped.
In the circularly polarized wave antenna device with the above-described features in accordance with the first aspect, since the surface shape of the radiation conductor is one wherein the electric lengths in two orthogonal directions of the radiation conductor are equal to each other, the surface of the radiation conductor is formed as a square by a visual observation, or as an electrical square wherein the electric lengths of two sides are equal. The diagonal directions of the square by a visual observation are orthogonal to each other. On the other hand, the electrical square is rectangular by a visual observation, but the diagonal directions of this rectangle by a visual observation are electrically orthogonal to each other.
By using this radiation conductor, the occurrence of the degeneration-separated mode generated when inputting a transmission power from the feeding conductor to the radiation conductor, is conditioned by the geometries of the radiation conductor and the capacitive loading conductors and the correlational positions therebetween. Specifically, by disposing capacitive loading conductors in the diagonal directions of the radiation conductor, and by making a difference between the capacitive loading conductors in the diagonal directions, an equivalent resonant circuit wherein a resonant current flow in each of the diagonal directions, is formed, and the directions in which resonant currents in the radiation conductor flow are determined. In other words, the degree that two electric fields (polarized waves) using resonant currents as an exciting source spatially intersect each other orthogonally, is determined.
Furthermore, by selecting the geometry of the capacitive loading conductors and the correlational positions between the radiation conductor and each of the capacitive loading conductors, the load capacitance values which vary in the capacitance value for every diagonal direction, are determined. The load capacitance constitutes a circuit element which determines the frequency difference between the two electric fields (polarized waves). In the radiation conductor in which the diagonal directions are orthogonal to each other, and which has a shape such that the electric lengths of two sides thereof are equal, two resonant currents in the degeneration-separated mode exhibit a phase difference of about 90xc2x0 therebetween, and the phase difference between the polarized waves also becomes about 90xc2x0.
As described above, in the present invention, since the phase difference between the two polarized waves can be made about 90xc2x0, and the polarized waves can be made to spatially intersect each other substantially orthogonally, it is possible to obtain an antenna which radiates circularly polarized electromagnetic waves from the radiation conductor.
Here, the xe2x80x9celectric length of the radiation conductorxe2x80x9d refers to the half length of an effective wavelength, in other words, a half length of the wavelength of an electromagnetic wave radiated from an antenna, divided by the root of the dielectric constant of the substrate. Also, the xe2x80x9cdegeneration separated modexe2x80x9d refers to exciting two resonant currents which have mutually different phases and frequencies, on the radiation conductor, by a single power feeding.
In the circularly polarized wave antenna device with in accordance with the second aspect, since the shape of the radiation conductor is formed into a square shape in a plan view or an electrical square shape in a plan view, and the capacitive loading conductors are provided so that the load capacitances are mutually different between the two diagonal directions, two resonant currents in the degeneration separated mode are excited by the power feeding from a single point except for the two diagonal directions, to the radiation conductor, as well as the directions in which the resonant currents flow are determined, and the polarized waves generated by these resonant currents spatially intersect each other substantially orthogonally. Also, the two resonant currents becomes ones which are mutually different in the resonant frequency and have a phase difference of about 90xc2x0 therebetween, and thereby the phase difference between the polarized waves having mutually different resonant frequencies, becomes about 90xc2x0.
The above-described resonant frequencies, in other word, the frequencies of polarized waves, are subjected to the influences of the load capacitances between the radiation conductor and each of the capacitive loading conductors, particularly the influence of the gap between the radiation conductor and each of the capacitive loading conductors, so that, by setting, to a desired value, the gap between the radiation conductor and each of the capacitive loading conductors and the geometry of the capacitive loading conductors, particularly the length and width, it is possible to set the frequency of polarized waves to meet a required antenna characteristic, and to select the frequency of electromagnetic waves to be radiated from the radiation conductor.
In the configuration wherein the substrate is formed into a hexahedron, and wherein the capacitive loading conductors having the same length in the same diagonal direction on the main surface, are provided along the edge lines of the side surfaces of the substrate, the operation of the degeneration-separated mode is determined by the structure of the antenna. Specifically, by disposing the capacitive loading conductors as closely along the edge lines of the side surfaces of the substrate as possible, two polarized waves having a phase difference of approximately 90xc2x0 can be made to spatially intersect each other substantially orthogonally. In addition, by forming the substrate as a hexahedron, the substrate can be formed to fit the shape of the substrate. Among alternatives, when a substrate having a square shaped main surface is adopted, the shape of the radiation conductor in a plan view and the shape of the main surface becomes the same, so that the substrate can be formed into the minimum size. In accordance with the present invention, the circularly polarized antenna device can be reduced in overall size.
The configuration of the capacitive loading conductors formed on the substrate can be determined in consideration of a required antenna characteristic, and consequently, the load capacitance corresponding to the frequency of the electromagnetic waves radiated from the radiation conductor. When forming each of the capacitive loading conductors by dividing them into a plurality of capacitive loading conductor pieces with gaps interposed therebetween, the load capacitance decreases, so that the frequency of the electromagnetic waves radiated from the antenna can be set to a high value.
In the configuration wherein, by extending the corners of the radiation, radiation conductor extension pieces are formed so as to extend downward to the side surface edges of the substrate, the load capacitance is mainly formed between each of the radiation conductor extension pieces and one of the capacitive loading conductor pieces, and desired load capacitance can be set by setting the gap.
In the configuration wherein the capacitive loading conductor is extended to the main surface on which the radiation conductor is formed, since the load capacitance or the electrostatic capacitance between the radiation conductor and each of the capacity loading conductors becomes large, the frequency of electromagnetic waves radiated from the radiation conductor can be reduced. Also, in the configuration wherein the capacitive loading conductors are formed into a meander shape, an inductance component can be added in addition to a capacitive component when attempting to determine the resonant frequency of the two currents in the degeneration-separated mode, in other words, the frequency of two polarized waves. In any of the above-described cases, if the substrate is formed as a rectangular parallelepiped, and the capacitive loading conductor is formed so as to have a small width, circularly polarized waves in the degeneration-separated mode which spatially intersect each other substantially orthogonally, and which have a phase difference of about 90xc2x0 therebetween, will be ensured.
The above and other objects, features, and advantages of the present invention will be clear from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings.